This invention relates to compressors and more particularly it is concerned with a multi-vane type compressor.
Generally, one of the main losses of powers in a multi-vane type compressor is a friction loss between the tips of the vanes and the inner peripheral surface of the cylinder. If this loss could be reduced, a great reduction in the losses of power would be realized, thereby contributing to improvements in energy efficiency ratio (EER) in a refrigerating machine.
To reduce the friction loss occurring between the tips of the vanes and the inner peripheral surface of the cylinder requires a reduction in forces urging the vanes to move toward the inner peripheral surface of the cylinder. However, if the force urging the vanes toward the inner peripheral surface of the cylinder is low, jumping action would occur when the vanes move in sliding movement along the inner surface of the cylinder, thereby causing noise to be produced and wear and damage to occur. Conversely if the force urging the vanes toward the inner peripheral surface of the cylinder is too high, friction loss would be great and the loss of power would also be great, thereby giving rise to problems that are contradictory in solution.
The force F.sub.WA urging the vanes toward the inner peripheral surface of the cylinder can be expressed by the following equation: EQU F.sub.WA =F.sub.IE +F.sub.CE +F.sub.BA
where F.sub.IE is the inertial force applied to the vanes, F.sub.CE is the centrifugal force applied to the vanes and F.sub.BA is the back pressure applied to the vanes. To avoid the value of F.sub.WA becoming too high, F.sub.IE and F.sub.CE could be reduced by selecting suitable materials for the vanes and altering the dimensions thereof. With regard to F.sub.BA, it is necessary to reduce the vane back pressure. However, the internal pressure of the cylinder rises as compression progresses, and the force tending to force the vanes backwardly rises, thereby causing jumping action to readily occur. The result of this is that it is necessary to reduce the vane back pressure in the low pressure zone (suction and compression stroke zone) in which the internal pressure of the cylinder is low and to increase the vane back pressure in the high pressure zone (discharge stroke zone) in which the internal pressure of the cylinder is high.
In order to meet these requirements, proposals have hithereto been made to introduce into vane back pressure spaces defined between a plurality of vane grooves formed in the rotor and the bottom surfaces of the vanes slidably fitted in the respective vane grooves, a suction gas pressure in the suction and compression strokes and a discharge gas pressure in the discharge stroke, as disclosed in Japanese Utility Model Application Laid-Open No. 106391/80, for example.
Generally toward the end of the high pressure zone (discharge stroke zone), overcompression takes place in the internal pressure of the cylinder that is higher than the discharge gas pressure. Thus in the proposals referred to hereinabove, the problems have arisen that the jumping action of the vanes is caused to occur by the overcompression and the compressor vibrates, causing noise level to rise.
To avoid this trouble, proposals have been made to introduce into the vane back pressure spaces a pressure of higher level through the entire zone of the discharge stroke, as disclosed in U.S. Pat. No. 2,827,226 granted to Alex A. McCormack, for example. However, an unnecessarily high back pressure is applied to the vanes in the initial zone of the discharge stroke; thereby increasing the friction loss at the tips of the vanes.
Another problem encountered with respect to a multi-vane type compressor concerns lubrication. The tips of the vanes move at high speed in sliding movement while being forced against the inner peripheral surface of the cylinder. This makes good lubrication of this part to be effected difficultly so that there has hitherto been a tendency that friction loss is high, loss of powers in high and seizure and galling are likely to occur. For example, when a multi-vane type compressor of an elliptic shape having a cylinder of a major radius of 35 mm, a minor radius of 30 mm and a thickness of 28.5 mm and vanes of a thickness of 2 mm is operated with a chlorofluorocarbon refrigerant R-22 at a high pressure 20 atg and a low pressure 6 atg, the oil film formed at the tip of each vane is 0.3-1.4 .mu.m, with a mean of about 0.5 .mu.m. Meanwhile the roughness of the surface of the inner periphery of the cylinder is limited to 0.5-1.0 .mu.m when finishes are given by ordinary machining, so that when the oil film has the aforesaid thickness (a mean value of about 0.5 .mu.m), the tip of each vane would be brought to metal-to-metal contact with the inner peripheral surface of the cylinder. This would cause wear to develop on the inner peripheral surface of the cylinder and increase friction work, causing a detrioration in energy efficiency. That is, in such mixed lubrication region, the coefficient of friction (C.sub.F) is about 0.02-0.08, with the value of C.sub.F becoming too large.
Meanwhile if it is possible to finish the inner peripheral surface of the cylinder in a manner to reduce the surface roughness below 0.5 .mu.m, fluid lubrication could be achieved and friction loss would be greatly reduced, because the coefficient of friction C.sub.F is about 0.001. However, fine finishing is very expensive and not economical.